1. Field of the Invention
The present invention relates generally to communication networks, and more specifically, to ATM (Asynchronous Transfer Mode) equipment having module redundancy for protecting network connections from failure.
2. Description of Related Art
ATM (Asynchronous Transfer Mode) high-speed switched networks have been proposed as a technology capable of integrating present digital services with new multimedia services, e.g., video on demand, live television from many sources, CD-quality music, LAN interconnection, and high-speed data transport for science and industry. To implement these different services, ATM networks are designed to handle a variety of traffic characteristics, e.g., constant rate, variable rate, and bursty, required for various real time, such as video conferencing, and non-real time, such as E-mail and web browsing, applications. The basic principles of ATM networks are well known to those skilled in the art and are discussed in numerous published references (see, for example, Andrew S. Tanenbaum, Computer Networks, Prentice Hall, 1996, 3rd ed.).
ATM switches transfer data using a cell switching technology. All data in an ATM network is transmitted between the switches of the network in small, fixed size 53 byte long ATM cells having 5 bytes for a header and 48 bytes for the information or data payload. The header of each cell contains a virtual connection identifier used for routing the cells over the network. Each ATM cell contains a two-part connection identifier in the cell header: a Virtual Path Identifier (VPI) and a Virtual Circuit Identifier (VCI). This two-part connection identifier uniquely identifies an ATM virtual connection on a physical interface.
ATM is a connection-oriented technology. That is, a call is required by user A to user B, similar to a telephone call, to set up a connection between the two users. Once the connection path is established, all data cells are transferred over the same connection path. This guarantees that cells will be delivered in order. Although ATM establishes a circuit between hosts, it establishes this circuit internally using cell switching technology. When a virtual circuit is established between hosts, what really happens is that table entries storing routing information are made in each switch along the path between the hosts.
The following is an example of what happens when a user A connects to a user B. First, user A sends a call request to user B. The call request arrives at an ingress switch of the ATM network. Call control software located at the ingress switch uses NNI signaling procedures combined with NNI routing services to locate user B and establish a virtual connection over which user A and user B exchange data.
Once a connection is established, data can be exchanged between User A and User B. Typically, the flow of data proceeds as follows. User A opens an application on the computer of User A which sends data to User B. Network software at User A's side will receive the data from the application of user A, and arrange the data into packets. Each packet is arranged as a variable length sequence of 53 byte ATM cells carrying the data payload preceded by a packet header cell containing information related to the application. The packets are sent over the ATM network cell by cell from User A to User B. The cells are reassembled into packets by software at User B's end and presented to User B as an application packet.
ATM networks provide several mechanisms for assuring quality of service and regulating traffic flow and congestion. Traffic shaping and traffic congestion control have been proposed for ATM networks. In traffic shaping, a quality of services contract is formed before data transmission between the customer and the network where the required transmission parameters are specified. For example, a customer could specify a requirement that data be transferred with a delay of 10 microseconds. In turn, the customer is required to abide by conditions of the contract, e.g., to transmit below an agreed upon rate. Traffic policing functions, e.g., user parameter control (UPC) within the ATM network enforce the user's agreement to the terms in the contract.
The physical layer connection of ATM networks normally consists of optical fibers working on the SONET (Synchronous Optical Network) standard. A SONET system consists of transport network elements such as add/drop multiplexers (ADMs), multiplexers, and repeaters all connected by optical fiber. An ATM switch in a SONET network may have one or more SONET interface unit boards which extract ATM cells from the optical signals of the fiber pathways to allow the ATM processing and routing functions to be applied by the ATM switch.
In ATM networks, the reliability of the ATM switch is critical because malfunctioning ATM equipment may result in the failure of an individual permanent virtual circuit (PVC) and a switched virtual circuit (SVC) connections. Malfunctions of the ATM equipment are generally classified into two types of failures: facilities failure and equipment failure. A facility failure refers broadly to the failure of anything external to the equipment such as a failure of one of the cables coming into the equipment, e.g., a fiber cut, or even connector problems. Someone could unplug a connector on the front of the equipment and that would be classified as a facility failure. An equipment failure includes any failure occurring within the electronics of the equipment itself (e.g. electronic failure).
It would be highly desirable to provide protection for ATM equipment or other communication switching equipment in order to preserve network reliability despite the above stated facility and equipment failures.